The present invention relates to platform stabilization systems, and more particularly relates to an improvement in passive stabilization systems suitable for satellite tracking in maritime applications or the like.
Shipboard maritime communication systems impose many requirements on satellite tracking apparatus. Tracking antennas installed on the ship must first acquire the desired target satellite in stationary earth orbit. Once the target satellite has been acquired, the orientation of the antenna must be continually updated for changes in the ship's heading and the ship's position. This is accomplished by controlling the position of the antenna in the elevation and azimuth directions. Changes in the ship's heading are detected by a gyro compass. The platform supporting the antenna is usually automatically responsive to the gyro compass and driven in the azimuth direction in order to compensate for changes in the ship's direction. The ship's position changes are generally updated manually or automatically.
In maritime satellite communication systems, two primary ship motion disturbances, pitch and roll, must be considered. These motions require that the antenna control system automatically compensate for angular changes quickly and precisely in order to avoid excessive error in antenna orientation. Conventional passive antenna stabilization systems include two flywheels or gyros in order to attenuate roll and pitch motion independently. This is accomplished by allowing the gyros to precess through a limited angular displacement without disturbing the primary pitch or roll axis.
Examples of prior art passive stabilization systems are found in U.S. Pat. Nos. 4,020,491 and 4,118,707. U.S. Pat. No. 4,020,491 discloses a gyro stabilized platform having one or more gyros mounted below the platform. The pivot axes of the gyro mounts disclosed therein are always perpendicular to the gimbal axes supporting the pivoted platform. U.S. Pat. No. 4,118,707 discloses a similar arrangement having a mechanism for shifting the center of gravity of the platform in order to achieve rapid adjustment of the position of the antenna.
Conventional passive stabilization systems suffer from certain serious drawbacks when the directional orientation of the platform is altered during pitch and roll disturbances. If the platform is rotated about the azimuth axis during pitch or roll motion, the resulting precession of one of the gyro axes will result in a horizontal torque component in accordance with the right hand rule for gyroscopic precession. The horizontal torque component will tend to tilt the platform during this motion. This horizontal torque component of gyro angular momentum prevents precise stabilization of the antenna platform and causes excessive tracking errors.
It is therefore an object of this invention to improve upon passive stabilization systems and overcome these problems.
Another object is to compensate for undesired torque influence thus providing a precise passive stabilization system.
A further object is to improve passive antenna stabilization systems suitable for maritime satellite communication systems.
Yet another object of this invention is to eliminate undesired torque influence in passive antenna stabilization system suitable for maritime communication when adjusting the position of the antenna during pitch or roll motion of a vessel.